Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens and trackpads are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. These devices can have a touch sensor panel, which can be a substantially clear panel with a touch-sensitive surface. Touch screens can also have a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. These devices can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, these devices can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event and can perform one or more actions based on the touch event.
Touch sensor panels can, in some embodiments, be formed from a matrix of drive lines (e.g., row traces) separated by a dielectric material from a plurality of sense lines (e.g., column traces), with sensors or pixels created at each crossing point of the drive and sense lines. Touch sensor panels can alternatively be arranged in any number of orientations or dimensions, including, but not limited to, diagonal, concentric circles, spiral, three-dimensional, or random orientations, in order to detect and identify the location of a touch on a touch sensor panel, stimulation signals can be provided to the drive lines, which causes charge to be coupled onto the sense lines. When a finger or other object disrupts this charge coupling, the sense lines generate signals indicative of touch output values. By knowing the timing of the stimulation signals provided to specific drive lines relative to the signals read out of the sense lines, processor(s) can be used to determine where on the touch sensor panel a touch event occurred.
In order for the touch sensors to correctly identify the presence and location of a touch event, the back plane of the touch sensor panel should be reliably grounded. Supplying a reliable ground connection for the back plane may be useful in providing a uniform electrical reference point to measure changes in voltage and capacitance due to a touch event. If the back plane is not properly grounded, the touch sensors may behave sporadically and misrepresent touch events or foil altogether.